1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) projection system and, more particularly, to an LCD projection system in which an image formed on a screen with curvilinear distortion and/or field curvature due to distortion aberration and curvature aberration generated in an optical system of a projection lens unit is corrected.
2. Description of the Related Art
An LCD projection system, which is one type of image projection system for displaying an image, generates an image using an LCD as an image generating means, and magnifies and projects the generated image onto a screen via a projection lens unit. The LCD projection system makes it easy to adjust the size of a screen and exhibits superior color reproducibility so that it is widely used as a large (over 40 inches) multimedia display device for HDTVs or video conferences.
LCDs are divided into a dispersion type such as a polymer dispersed liquid crystal (PDLC), and a polarization type such as twisted nematic (TN), according to a liquid crystal mode, and into a transmission type and a reflection type according to a transmission mode or a reflection mode of a light beam forming an image. Also, an LCD projection system is divided into a one panel type and a three panel type according to the number of LCD panels.
Referring to FIG. 1, a conventional one panel reflection type LCD projection system includes a light source 110 for generating and emitting light, a glass rod 130 for mixing incident light emitted from the light source 110 to emit light having a uniform intensity profile, a focusing lens 131, a collimating lens 132, a polarization beam splitter 140 for altering the optical path of incident light by transmitting or reflecting the incident light according to its polarization component, an LCD device 150 for generating an image by selectively reflecting incident light, and a projection lens unit 160 for magnifying and projecting incident light onto a screen S.
The light source 110 consists of a lamp 111 for generating light and a reflection mirror 112 such as a parabolic mirror or an elliptical mirror for reflecting the light emitted from the lamp 111 and guiding the reflected light along a predetermined proceeding path.
Reference numeral 120 denotes a color wheel which is rotatably installed on the proceeding path of light emitted from the light source 110. The color wheel 120 has red (R), green (G) and blue (B) color filters equally arranged on the surface thereof so that light of a particular color of the light emitted from the light source 110 is selectively transmitted.
The glass rod 130 mixes incident light by irregularly reflecting the light to emit light having a uniform intensity profile. The focusing lens 131 focuses and diverges the light passing through the glass rod 130 to enlarge the transmission width of the light. The collimating lens 132 collimates the diverging incident light to make a parallel beam.
The polarization beam splitter 140 is arranged on the optical path between the collimating lens 132 and the LCD device 150. The polarization beam splitter 140 alters the proceeding path of incident light by transmitting or reflecting the incident light by a mirror surface 141 according to its polarization component (P polarization or S polarization). The light passing through the polarization beam splitter 140 is incident upon the LCD device 150 and reflected by the LCD device 150 with the polarization direction being rotated by 90xc2x0. The light reflected by the LCD device 150 is incident upon the polarization beam splitter 140 again and totally reflected by the mirror surface 141. The totally reflected light passes through the projection lens unit 160 and is magnified and projected onto the screen S.
FIG. 2 shows an optical structure of a conventional one panel transmission type LCD projection system. Referring to the drawing, a transmission type LCD device 250 is arranged on the proceeding path of light. The LCD device 250 generates an image by selectively driving individual pixels to determine transmission of the light emitted from a light source 210. Reference numerals L1, L2 and L3 denote optical lenses for forming a parallel light beam. Reference numeral M denotes a reflection mirror for altering the proceeding path of light so that the light proceeds toward a projection lens unit 260 and a screen S.
In the above LCD projection systems, optical aberration is usually generated when an image is formed on the screen S via the projection lens unit (160 or 260). That is, optical aberration such as curvilinear distortion or field curvature is generated.
As is well known, the distortion is generated by lens aberration, which refers to the focal distance at points near the optical axis and far from the optical axis being different. That is, with respect to an ideal state where no distortion is generated, such as an image surface 31 as shown in FIG. 3A, a positive (+) distortion image 32 (pin-cushion distortion), having concave sides as shown in FIG. 3B, or a negative (xe2x88x92) distortion image 33 (barrel distortion), having convex sides as shown in FIG. 3C are generated.
The positive (+) distortion image 32, as shown in FIG. 3D, means that a square image generated by the flat type LCD device 150 (250) is distorted to positive (+) due to aberration of a lens 160a (260a) and formed on the screen S by being distorted in a pin-cushion shape. The negative (xe2x88x92) distortion image 33, as shown in FIG. 3E, means that a square image generated by the flat type LCD device 150 (250) is distorted to negative (xe2x88x92) due to aberration of a lens 160b (260b) and formed on the screen S by being distorted in a barrel shape.
The above-mentioned curvature distortion is caused by lens aberration which forms an image to focus on a curved (concave or convex) surface with respect to a flat screen S, as shown in FIG. 4. Due to the curvature distortion, a flat image generated by the flat type LCD device 150 (250) is formed as an image 34 in a state of field curvature.
FIG. 4 shows a case in which the surface of the image formed on the screen S is concave due to lens aberration. Although not shown in the drawing, the surface of the image may be formed on the screen S to be convex due to lens aberration. The curvature mentioned below includes both the concave and convex states.
In the conventional LCD projection system, the distortion and curvature are generated in combination in an actual projection lens unit optical system to make an image light having a plane shape generated by the flat type LCD device 150 (250) to be an image having the sides curved and simultaneously the surface curved (concave or convex). Finally, the image becomes a complexly distorted image.
Also, the projection lens unit 160 (260) generates chromatism according to the wavelength of incident light. Due to chromatism, a red ray, of which the wavelength is relatively longer than other rays incident in parallel on an optical lens (convex lens) of the projection lens unit 160 (260) is focused at a point beyond the focal distance of the optical lens (convex lens). A blue ray, of which the wavelength is relatively shorter than other rays incident in parallel on an optical lens (convex lens) of the projection lens unit 160 (260), is focused at a point located closer than the focal distance of the optical lens (convex lens). Thus, the red, green and blue color images formed by the LCD device are not formed to be the same size on the screen. In this case, differences in the magnification power of red, green and blue color images are generated so that mismatch among the red, green and blue color images gradually increases from the center to the periphery of the screen, lowering the quality of the image.
Thus, in the LCD projection system, the projection lens unit 160 (260) needs an optical configuration designed to perform an optimal correction considering both chromatism and optical lens aberration such as distortion aberration and curvature aberration. However, it not difficult to perfectly correct chromatism and distortion aberration and curvature aberration because of properties of an optical lens.
Practically, various types of optical lenses are added to or excluded from the projection lens unit for correction of the chromatism and distortion aberration and curvature aberration, so that the projection lens unit is designed and manufactured in the state in which the chromatism and distortion aberration and curvature aberration are appropriately corrected. However, the above correction method has limits according to specifications of the projection lens unit, a complex optical configuration and aberration of other optical lenses.
To solve the above problems, it is an objective of the present invention to provide an LCD projection system which can correct an image formed to be distorted and/or curved on a screen due to distortion aberration and/or curvature aberration of optical lenses of a projection lens unit.
Accordingly, to achieve the above objective, there is provided an LCD projection system including a light source for generating and emitting light, an LCD device for generating image light using incident light emitted from the light source, and a projection lens unit for magnifying and projecting the image light emitted from the LCD device onto a screen, in which the LCD device has a distorted and/or curved shape corresponding to an image formed on the screen before correcting for distortion and/or curvature of the image formed on the screen, which are generated due to distortion aberration and/or curvature aberration of the projection lens unit, so that distortion and/or curvature can be corrected.